handle the stress tag in shift_energy_to_zero.py

[MoseyQAQ]

Summary
Fixes an issue in shift_energy_to_zero.py where stress data was dropped when processing extxyz files.

This clear will also delete the attached stress data:

GPUMD/tools/Analysis_and_Processing/shift_energy_to_zero/shift_energy_to_zero.py

Line 93 in 52b9e6f

all_frames[i].calc = None

Modification
This change explicitly retrieves and saves the stress to atoms.info (if available) before the calculator is reset.

    try:
        stress = all_frames[i].get_stress(voigt=False)
        all_frames[i].info['stress'] = stress
    except:
        pass